You'll have to be a bit more specific on what's happening and how. It could be a world of things like a windows update.
If you have a date when everything worked peachy, and you have a restore point to that, I would start with that.
Check temps, get some monitoring tools and stress test.
Just a remark, that's a pretty old system right there, don't expect it to be a fine wine. Hardware won't age well and new software will be more demanding on your parts.
Good luck
The Englishman